1. Field of the Invention
Embodiments of the invention generally relate to diesel fuel combustion systems, processes and compositions and, more particularly, to a near-supercritical liquid or supercritical diesel fuel composition, combustion process for the composition and fuel system for the combustion process.
2. Description of Related Art
Recently enacted clean air legislation has targeted fossil fuel emissions. In particular, diesel engine manufacturers and fuel providers are under pressure to develop new solutions for cleaner and more efficient combustion in diesel engines. Engine designers are having to rethink the entire combustion process from beginning to end.
Although fuel combustion is among the oldest known processes to transform chemical energy into useful heat/work/power, two major problems are still associated with combustion technology: relatively low energy efficiency and pollution. Successive incremental improvements to classical diesel fuel combustion processes have reached a plateau. Making further improvements in both efficiency and emissions performance has become increasingly limited. Consequently, greater attention needs to be given to fuel formulations and improved combustion processes.
Diesel fuels are complex mixtures of alkanes, alkenes, and aromatic hydrocarbons (>100 components). Biodiesel is a diesel fuel substitute/additive composed of methyl/ethyl esters that are produced from vegetable oils (e.g. soybean, rapeseed) or animal fats. Both of these, as well as other blended diesel fuel mixtures are referred to herein as diesel fuel. Many diesel fuel compounds generate harmful combustion products (e.g., NOx, SOx) due to high combustion temperatures, while other fuel components are incompletely oxidized primarily to CO, aldehydes, and polyaromatic hydrocarbons (PAHs). The generation of these components and other particulate matter pose both health and environmental concerns. The products of the incomplete oxidation process are ultimately due to diesel fuel injection as liquid droplets in the combustion chamber of the engine. Despite the use of preheating, additives, and higher injection pressures to generate micro droplets (around 10 μm diameter range), these problems persist. Even these small droplets fail to completely evaporate and combust. Preheating the fuel provides better fuel-air mixing, however this process is limited to temperatures in the range of about 150-200° C. due to coke, gum and tar formation at higher temperatures. Moreover, a decrease in particulate matter emissions produce an increase in NOx and SOx content, and vice versa, challenging the success of meeting ever tighter emission requirements.
Various fuel injection and combustion systems, and fuel compositions are described in the art. By way of example, U.S. Pat. No. 6,010,544 describes a supercritical (SC) composition of matter being a mixture of 5-50% water and a hydrocarbon. The patent discloses experiments performed with this SC composition demonstrating that a cleaner combustion can occur compared to that of subcritical phase diesel fuels. The advantages of this cleaner combustion, however, are offset by the necessary inconvenience of a water tank in addition to the fuel tank and the fact that water requires high energy to be brought to SC conditions. Further, the water is corrosive and its immiscibility with diesel fuel renders its use as a diluent unattractive for diesel engines. US 2002/0088168 discusses the use of gaseous carbon dioxide to form a mixture with low vapor pressure fuels at normal pressures and temperatures. The limited presence of CO2 in diesel fuel is helpful in facilitating the production of micro droplets that enhance combustion in diesel engines resulting in a reduction of particulate matter. However, the amounts of CO2 in the liquid and vapor phases of the proposed composition are affected by mechanical vibrations and variations in pressure and temperature of the fuel stream. More significantly, fuel injection in the form of droplets and the associated problems with droplet combustion persist. WO02/095210 proposes a method for fuel injection in a 4-stroke diesel engine wherein a first portion of the injected fuel is combined with hot exhaust gas to vaporize some of the diesel fuel prior to injection in a homogeneous charge compression ignition (HCCI) process. Control of the ignition timing is a problem associated with a HCCI process because the fuel is injected before the piston is at top dead center position to allow more time for fuel droplets to vaporize. Thus the process proposed under the limited HCCI conditions may not result in complete fuel vaporization. The foregoing references are hereby incorporated by reference in their entirety to the fullest allowable extent.
In light of the foregoing, the inventors have recognized a need for diesel fuel compositions, fuel oxidation processes and fuel combustion systems that overcome the cited disadvantages and others known in the art. In addition, improvements in performance, efficiency, engine life and operating cost reductions are exemplary benefits of the invention embodiments described in detail below.